The present invention concerns a method for injecting fuel into a burner, for example into a double cone burner, as well as a burner for performing this method.
Frequently, so-called thermoacoustic fluctuations occur in burners that supply liquid or gaseous fuel to a combustion chamber where the fuel burns on a flame front. This is also the case, for example, with the very successfully used, so-called double-cone burner as described in EP 0 321 809. In addition to fluidic stability, mixture break fluctuations are a main reason for the occurrence of such thermoacoustic instabilities. Fluid-mechanical instability waves generated at the burner result in a formation of whirls (coherent structures) that influence the combustion and may lead to a period heat release and pressure fluctuations associated with it. The fluctuating air column in the burner results in fluctuations in the mixture break with the respective associated fluctuations in the heat release.
These thermoacoustic vibrations present a risk for any type of combustion application. They result in high-amplitude pressure vibrations, a limitation of the operating range, and may increase noxious emissions. This is true in particular for combustion systems with low acoustical attenuation. In order to permit a high performance conversion over a broad operating range with respect to pulsations and emissions, an active control of the combustions vibrations may be necessary.
Coherent structures play a critical role in the mixing processes between air and fuel. The dynamics of these structures therefore influence the combustion and therefore the heat release. A control of the combustion instabilities is made possible by influencing the shear layer between the fresh gas mixture and recirculated waste gas (for example, Paschereit et al., 1998, xe2x80x9cStructure and Control of Thermoacoustic Instabilities in a Gas-turbine Burnerxe2x80x9d, Combustion, Science and Technology, Vol. 138, 213-232). One possibility for doing this is acoustic excitation (EP 0 918 152 A1).
The flame position can be changed by fuel staging, and the influence of flow instabilities as well as of time-lag effects can be reduced.
A further mechanism that may result in thermoacoustic vibrations are fluctuations in the mixture break between fuel and air.
The invention therefore has the objective of disclosing a burner for performing such a method in which the occurrence of such thermoacoustic vibrations is reduced or even avoided.
This concerns a method for injecting fuel into a burner comprising an inner chamber enclosed by at least one shell, at which inner chamber fuel is injected through fuel nozzles into a combustion air stream flowing inside the inner chamber, the resulting fuel/air mixture flows within a time-lag xcfx84 to a flame front in a combustion chamber, and is ignited there.
According to the invention, thermoacoustic fluctuations are reduced or even avoided altogether with such a method in that the fuel is injected by means of fuel nozzles distributed over the burner length in such a manner that the time-lag xcfx84 between the injection of the fuel and its combustion at the flame front corresponds to a distribution that varies systematically for the various fuel nozzles and prevents ignition-driven vibrations.
According to experience, in a conventional burner the time-lag xcfx84 between the injection site and the effective combustion at the flame front is essentially identical for all of the burner nozzles distributed over the burner length. An unsystematic, slight variation from the injection position around a mean value is found. As a result, it is easy for thermoacoustic vibrations to form. The core of the invention therefore consists of injecting the fuel into the combustion air stream in such a way that no time-lag xcfx84 between the injection site and the effective combustion at the flame frontxe2x80x94a time-lag that is essentially identical for all fuel nozzles distributed over the burner lengthxe2x80x94occurs, but that the time-lag assumes a distribution that systematically varies over the burner length.
A first preferred embodiment of the invention is characterized in that the maximum time-lag xcfx84max between injection site and flame front is in the range of xcfx84max=5-50 ms, and that, especially preferred, with a flow speed of the fuel/air mixture in the inner chamber in the range from 20-50 m/s, the maximum time-lag xcfx84max is in the range of xcfx84max=5-15 ms, and this with consideration of the shifting of the flame front position in relation to the flow speed. If the method is used under such conditions, thermoacoustic vibrations can be reduced especially well.
In another embodiment of the invention, the fuel is injected in such a manner that the time-lag distribution over the burner length towards the burner end is designed so as to essentially decrease in a linear manner from the maximum value xcfx84max by a maximum time-lag differential xcex94xcfx84 towards a minimum value at the burner end of xcfx84maxxe2x88x92xcex94xcfx84. This simple distribution can be realized with relatively little expenditure and has an efficient effect. It is found that the time-lag differential xcex94xcfx84 is preferably set in the range from 10-90% of the maximum value xcex94max, especially in the range above 50% of the maximum value xcfx84max.
The burner in another embodiment of the method is a double cone burner, in which the burner is made up of at least two superimposed hollow partial cone bodies that are provided in the flow direction with an increasing cone angle, and which partial cone bodies are arranged offset in relation to each other so that the combustion air flows through a gap between the partial cone bodies into the inner chamber. The method can be used especially advantageously in this already mentioned, premix-like double cone burner.
The invention furthermore concerns a burner for performing the above method, whereby the fuel nozzles are divided into groups, and whereby in each case one group of fuel nozzles are arranged on a line in such a manner that all fuel nozzles of a group are responsible for feeding the same area in the flame front. It is especially preferred that with such a burner the fuel nozzles are distributed in such a manner that the number of lines is greater than the average number of fuel nozzles of a group. For example, in a double cone burner the fuel nozzles on the cone surfaces of the partial cone bodies can be arranged on lines for an area of the flame front. It is hereby found that a division of the overall 32 nozzles of a double cone burner into 8 groups on 8 lines with 4 each nozzles is advantageous.